The present invention relates to a control system for accurately measuring the draft force exerted by a draft implement such as a plow or cultivator. In another aspect of the invention, the control system adjusts the depth of the draft implement in response to the measurement of draft load.
It is known to adjust the depth of a draft implement in inverse relation to a measurement of draft load. One of two reasons are typically given for this adjustment. One reason is that without depth adjustment, the vehicle engine may stall or incur damage when an excess draft load is encountered, such as when encountering heavy soil conditions during field plowing. Another reason is that without depth adjustment, the depth of the draft implement increases when the towing vehicle encounters an incline and, conversely, decreases when the towing vehicle encounters a decline. By adjusting the depth of the draft implement in inverse relation to an indication of draft load, a more uniform plowed field may be obtained.
Several approaches are known for providing an approximation of draft load. In one prior approach, draft load is inferred from a deviation in measured engine rpm from a single set value of engine rpm associated with maximum horsepower output. A disadvantage with this approach is that draft vehicles are usually operated near the peak horsepower of the engine wherein the torque versus rpm curve is relatively flat. Substantially large variations in engine rpm are therefore associated with relatively small variations in draft load. Accordingly, this system is prone to fluctuate or search around a desired depth associated with the set value of engine rpm. Thus, even without variations in soil density or variations in grade, the plowed field depth will fluctuate.
In another approach, as disclosed in U.S. Pat. No. 4,077,475, draft depth is varied in inverse relation to a combination of rpm deviation and actual implement depth. By adding feedback for actual implement depth, the effect of depth variation with variations in engine rpm is allegedly dampened. However, it appears that a system of this type is prone to oscillate. Further, the inventors herein have recognized that the degree of dampening is not adjustable to compensate for a change in the draft implement used or for changes in field soil conditions.
In another common approach, as disclosed in U.S. Pat. Nos. 3,834,481; 4,300,638; 4,343,365; 4,495,577, 4,518,044, a force transducer is coupled to a link arm of the tractor for providing an electrical signal which is related to the total force transmitted to the link arm. This signal is then used as a feedback variable in a feedback control system utilizing other feedback variables such as arm position, engine rpm, selected arm position, and hydraulic cylinder pressure. The inventors herein have recognized that a problem with providing a measurement of draft force from a force transducer coupled to a link arm is that the measurement of force includes more than just the draft force. For example, the force measurement will include a force component transmitted through a lift rod in raising the implement.